Transcript Semantic Web Technologies: The foundation for future
Semantic Web Technologies: The foundation for future enterprise systems
Okech Odhiambo Knowledge Systems Research Group Strathmore University 1
Outline
1. The Current Web and the Semantic Web 2. RDF and RDF Schema 3. Web Ontology Language OWL 4. Application areas for semantic web technology 5. Our reflections 2
Introduction
• In this talk we chose to look at the languages of the Semantic Web – Specifically RDF, RDF Schema and OWL – They are W3C recommendations • We believe that as web developers, these languages are a necessary part of the skill set required of you 3
The Current Web
• Content is designed for human consumption. • Main focus is on documents – a document or media can link to any other document (or different media). 4
The Semantic Web
• An extension of the current web in which information is given a well defined meaning, better enabling computers and people to work in cooperation The Semantic Web, Scientific American, May 2001 5
The Layer ‘Cake’
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Semantic Web Languages
• Resource Description Framework (RDF) – RDF is a language ( + XML syntax + semantics) for representing metadata – for describing the semantics of information in a machine- accessible way • RDF Schema (RDFS) extends RDF with “schema vocabulary” – Class, Property – type, subClassOf, subPropertyOf – range, domain – RDFS is a very simple ontology language 7
The RDF Data Model
Statements are (subject, predicate, object) triples: (fAcosta, hasWritten, “Research Methods in IT”) Can be represented as a graph:
hasWritten fAcosta subject predicate Research Methods in IT object
Statements describe properties of resources. A resource is any object that can be pointed to by a URI: Properties themselves are also resources (URIs) 8
Uniform Resource Identifier - URI
• "The generic set of all names/addresses that are short strings that refer to resources" – URLs (Uniform Resource Locators) are a particular type of URI, used for resources that can be accessed on the WWW (e.g., web pages) • In RDF, URIs typically look like “normal” URLs, often with fragment identifiers to point at specific parts of a document: – http://www.somedomain.com/some/path/file#fragmentID • Example : http://ks.strathmore.edu/example/#facosta – Shorthand notation strath:facosta 9
Linking Statements
The subject of one statement can be the object of another Such collections of statements form a directed, labeled graph http://ks.strathmore.edu/example/IT2145 http://ks.strathmore.edu/example/hasWritten http://ks.strathmore.edu/example/Title http://ks.strathmore.edu/example/facosta “Problem Based Learning Methodology” http://ks.strathmore.edu/example/hasName “Freddie Acosta” http://ks.strathmore.edu/example/homePage http://ks.strathmore.edu/example/email http://www.strathmore.edu/lecturers/facosta mailto:[email protected]
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RDF Syntax
• RDF has an XML syntax – Every
Description
element describes a resource – Every attribute or nested element inside a
Description
is a
property
of that Resource
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RDF Schema
• RDF gives a language for meta data annotation, and a way to write it down in XML, but it does not provide any way to structure the annotations • RDF Schema augments RDF to allow you to define vocabulary terms and the relations between those terms – it gives “extra meaning” to particular RDF predicates and resources – e.g., Class, subClassOf, Property, domain, range • These terms are the RDF Schema building blocks (constructors) used to create vocabularies 12
RDF Schema - Classes
rdfs:Type strath:Publication rdfs:subClassOf rdfs:Class strath:Article rdf:subClassOf strath:JournalArticle rdf:Type strath:IT2145
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RDF Schema - Properties
rdfs:subClassOf strath:Faculty rdfs:Domain rdfs:Property rdfs:Type strath:Employee rdfs:Type rdfs:Class strath:hasWritten rdf:Range rdfs:Type strath:Publication
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RDF Summary
• RDF - The Resource Description Framework allows us to describe resources by specifying their properties and property values.
– RDF Statements are triples of the form (Subject, Predicate, Object) – A set of RDF triples forms an RDF Graph • RDF Schema semantically extends RDF by providing a means to describe RDF Vocabularies.
• RDF and RDF Schema provide basic capabilities for describing vocabularies that describe resources.
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Problems with RDF Schema
• RDFS too weak to describe resources in sufficient detail – No localised range and domain constraints • Can’t say that the range of hasChild is person when applied to persons and elephant when applied to elephants – No existence/cardinality constraints • Can’t say that all instances of person have a mother that is also a person, or that persons have exactly 2 parents – No transitive, inverse or symmetrical properties • Can’t say that isPartOf is a transitive property, that hasPart is the inverse of isPartOf, or that touches is symmetrical – Need to extend RDFS to provide better support for ontologies • Difficult to provide reasoning support 16
Ontology
• Borrowed from philosophy - the study of “The nature of being” • “An ontology is a formal, explicit specification of a shared conceptualization specification of a conceptualization” (Gruber, 1993) • In general, an ontology provides a mechanism to capture information about the objects and the relationships that hold between them in some domain of interest.
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An Ontology Language for the Semantic Web
• Create a richer ontology language for the Semantic Web • Desirable features identified for Web Ontology Language: – Extend existing Web standards • Such as XML, RDF, RDFS – Easy to understand and use – Formally specified • Possible to provide automated reasoning support 18
History
• Two languages were developed to satisfy above requirements – OIL: developed by group of (largely) European researchers – DAML-ONT: developed by group of (largely) US researchers • Efforts merged to produce DAML+OIL – Done by a “Joint EU/US Committee on Agent Markup Languages” • DAML+OIL was submitted to W3C as basis for standardization – Web-Ontology (WebOnt) Working Group developed OWL language based on DAML+OIL – OWL language now a W3C Recommendation 19
OWL Overview
• OWL is an ontology language, based on ideas from Description Logics • Well defined semantics • OWL extends the expressive power of RDFS – Can talk about defined properties of classes OWL Description Logics RDF/RDFS XML Schema 20
Components of an OWL Ontology
• Individuals • Classes • Properties Country Kenya Philippines UK isCitizenOf Acosta Publication hasColleague Clement Research Methods in IT hasWritten Advanced Accounting Employee 21
The Three Species of OWL
• OWL-Full - No restrictions on how/where language constructs can be used. The union of OWL and RDF Schema OWL-Full is not decidable.
• OWL-DL - Restricted version of OWL-Full. Corresponds to a description logic. Certain restrictions on how/where language constructs can be used in order to guarantee decidability.
• OWL-Lite - A subset of OWL-DL. The simplest and easiest to implement of the three species.
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OWL Summary
• OWL is the latest standard in ontology languages.
• It is layered on top of RDF and RDFS, and has a rich set of constructs.
• There are three species of OWL: OWL-Lite, OWL-DL and OWL-Full.
• We can perform automated reasoning over ontologies that are written in OWL-Lite and OWL-DL.
• Tools to build Ontologies exists e.g. Protégé 23
Protégé
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SW Application Areas
• Application Areas – Knowledge Management • Focus of our research group: Knowledge Systems group – Enterprise Data Integration – Web services, to support • E-Commerce (B2C and B2B) – Context based search – E-Government – E0learning • … many more 25
Take Home Ideas
• Semantic web is an idea that will be with us • Standards have already be set by W3C • Tools are available for this 26
Questions?
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Layer Cake - Revised
W3C Semantic Web Language Layer Cake
revised version, Tim-Berners-Lee 2005
2000 Version 28